External and Internal Validity Paper

Describe evaluation in terms of Internal and External Validity.  Explain the threats/limitations of Internal and External Validity.  Provide an example of each of the threats/limitations of Internal and External Validity.

This the text book chapter 10

Dignan, M. B. Measurement and Evaluation of Health Education, 3rd edition. Springfield, IL: Charles C. Thomas. ISBN-13: 978-0398059583


Evaluation is the process of systematically collecting and analyzing data to determine the effectiveness of a program or intervention. Evaluation can be assessed in terms of internal and external validity.

Internal validity refers to the extent to which the study design and methods ensure that the observed effects are actually due to the intervention being studied and not to other factors. In other words, it measures the extent to which the study accurately reflects the intervention being tested.

External validity, on the other hand, refers to the extent to which the findings of a study can be generalized to other populations, settings, and time periods. In other words, it measures the extent to which the study’s results can be applied to other contexts.

Threats to internal validity include history, maturation, testing, instrumentation, regression to the mean, selection bias, and experimental mortality. History refers to external events that may influence the outcome of a study, maturation refers to natural changes that occur over time that may affect the outcome, testing refers to the effects of pretesting on the outcome, instrumentation refers to changes in measurement methods, regression to the mean refers to extreme values returning to the mean over time, selection bias refers to non-random assignment to groups, and experimental mortality refers to the loss of participants during the course of a study.

Threats to external validity include selection bias, reactive effects of experimental arrangements, multiple-treatment interference, and limited generalizability of findings to other settings, populations, and time periods.

For example, a study evaluating the effectiveness of a smoking cessation program may have a threat to internal validity due to testing if the pretest prompts participants to start thinking about quitting and influences their decision to participate in the program. A threat to external validity may occur if the study is conducted in a controlled setting with highly motivated participants and does not reflect the population of smokers in the real world.

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